Monitor configuration spoiled after reboot Plasma 5.10.2

I have a notebook (screen resolution 1600x900) and attached via HDMI a second monitor (Full HD)

When I set up my second screen properly so that the laptop screen is off and the second external monitor is my primary display and then do a reboot, the second screen becomes a second screen again without task bar or widgets, but also the notebook screen is this way. So I do not see my true first desktop working area as I would if I had just booted the notebook.

Only with Function + F8 hardware key combination for external monitor switching I can bring back my primary desktop image on the notebook, but then I have to reconfigure the second screen etc.

The other way is to unplug the second screen and then do a reboot (forced by "Power" hardware key as I have no access to start menu etc.)

This is very disturbing.

Can somebody please check that out?

My goal is to have laptop screen switched off and use the notebook with exteral screen so that after reboot the external screen is still the primary logical screen with task bar, menus and widgets.